The statements in this background section merely provide background information related to the present disclosure and may not constitute prior art.
Engineers typically design high-pressure oil field plunger pumps in two sections; the (proximal) power section and the (distal) fluid section. The power section usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. The power section is commonly referred to as the power end by the users and hereafter in this application. The fluid section is commonly referred to as the fluid end by the users and hereafter in this application. Commonly used fluid sections usually comprise a plunger pump housing having a suction valve in a suction bore, a discharge valve in a discharge bore, an access bore, and a plunger in a plunger bore, plus high-pressure seals, retainers, etc. FIG. 1 is a cross-sectional schematic view of a typical fluid end showing its connection to a power end by stay rods. FIG. 1 also illustrates a fluid chamber which is one internal section of the housing containing the valves, seats, plungers, plunger packing, retainers, covers, and miscellaneous seals previously described. A plurality of fluid chambers similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid end housing schematically illustrated in FIGS. 2A-D.
Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the valve body, which reversibly seals against the valve seat. In other applications, the term “valve” includes components in addition to the valve body, such as the valve seat and the housing that contains the valve body and valve seat. A valve as described herein comprises a valve body and a corresponding valve seat, the valve body typically incorporating an elastomeric seal within a peripheral seal retention groove.
Valves can be mounted in the fluid end of a high-pressure pump incorporating positive displacement pistons or plungers in multiple cylinders as illustrated in FIGS. 1 and 2. FIG. 3 illustrates a cross sectional view of one of the cylinders of FIGS. 1 and 2. Such valves typically experience high pressures and repetitive impact loading of the valve body and valve seat. These severe operating conditions have in the past often resulted in leakage and/or premature valve failure due to metal wear and fatigue. In overcoming such failure modes, special attention is focused on valve sealing surfaces (contact areas) where the valve body contacts the valve seat intermittently for reversibly blocking fluid flow through a valve.
Valve sealing surfaces are subject to exceptionally harsh conditions in exploring and drilling for oil and gas, as well as in their production. For example, producers often must resort to “enhanced recovery” methods to insure that an oil well is producing at a rate that is profitable. And one of the most common methods of enhancing recovery from an oil well is known as fracturing. During fracturing, cracks are created in the rock of an oil bearing formation by application of high hydraulic pressure. Immediately following fracturing, a slurry comprising sand and/or other particulate material is pumped into the cracks under high pressure so they will remain propped open after hydraulic pressure is released from the well. With the cracks thus held open, the flow of oil through the rock formation toward the well is usually increased.
The industry term for particulate material in the slurry used to prop open the cracks created by fracturing as the propend. And in cases of very high pressures within a rock formation, the propend may comprise extremely small aluminum oxide spheres instead of sand. Aluminum oxide spheres may be preferred because their spherical shape gives them higher compressive strength than angular sand grains. Such high compressive strength is needed to withstand pressures tending to close cracks that were opened by fracturing. Unfortunately, both sand and aluminum oxide slurries are very abrasive, typically causing rapid wear of many component parts in the positive displacement plunger pumps through which they flow. Accelerated wear is particularly noticeable in plunger seals and in the suction (i.e., intake) and discharge valves of these pumps.
A valve comprising a valve body and seal assembly and valve seat that is an example full open design valve and seat for a fracturing plunger pump is schematically illustrated in FIG. 4. FIG. 5 shows how propend particulates, such as sand and/or aluminum oxide spheres, may become trapped between sealing surface of valve body and sealing surface of valve seat as the suction valve closes during the pump's pressure stroke.
The valve of FIG. 4 is shown in the open position. FIG. 5 shows how accelerated wear begins shortly after the valve starts to close due to back pressure. For the valve, back pressure tends to close the valve when downstream pressure exceeds upstream pressure. For example, when the valve is used as a suction valve, back pressure is present on the valve during the pump plunger's pressure stroke (i.e., when internal pump pressure becomes higher than the pressure of the intake slurry stream). During each pressure stroke, when the intake slurry stream is thus blocked by a closed suction valve, internal pump pressure rises and slurry is discharged from the pump through a discharge valve. For a discharge valve, back pressure tending to close the valve arises whenever downstream pressure in the slurry stream (which remains relatively high) becomes greater than internal pump pressure (which is briefly reduced each time the pump plunger is withdrawn as more slurry is sucked into the pump through the open suction valve).
When back pressure begins to act on a valve, the propend slurry particles become trapped in the narrow space that still separates the sealing surfaces of the valve body and seat. This trapping occurs because the valve is not fully closed, but the valve body's elastomeric seal has already formed an initial seal against the valve seat. The narrow space shown in FIG. 5 between metallic sealing surfaces of the valve body and valve seat respectively is typically about 0.040 to about 0.080 inches wide; this width (being measured perpendicular to the sealing surfaces of the valve body and seat) is called the standoff distance. The size of the standoff distance is determined by the portion of the valve body's elastomeric seal that protrudes beyond the adjacent valve body sealing surfaces to initially contact, and form a seal against, the valve seat. As schematically illustrated in FIG. 5, establishment of this initial seal by an elastomeric member creates a circular recess or pocket that tends to trap propend particulate matter in the slurry flowing through the valve.
Formation of an initial seal as a valve is closing under back pressure immediately stops slurry flow through the valve. Swiftly rising back pressure tends to drive slurry backwards through the now-sealed valve, but since back-flow is blocked by the initial valve sealing, pressure builds rapidly on the entire valve body. This pressure acts on the area of the valve body circumscribed by its elastomeric seal to create a large force component tending to completely close the valve. For example, a 5-inch valve exposed to a back pressure of 15,000 pounds per square inch will experience a valve closure force that may exceed 200,000 pounds.
The large valve closure force almost instantaneously drives the affected valve, whether suction or discharge, to the fully closed position where the metal sealing surface of the valve body contacts the corresponding metal sealing surface of the valve seat. As the valve body moves quickly through the standoff distance toward closure with the valve seat, the elastomeric seal insert is compressed, thus forming an even stronger seal around any slurry propend particles that may have been trapped between the seal insert and the valve seat.
Simultaneously, the large valve closure force acting through the standoff distance generates tremendous impact energy that is released against the slurry particles trapped between the metallic sealing surfaces of the valve body and the valve seat. As shown in FIG. 6, the slurry propend particulates that are trapped between approaching valve sealing surfaces are severely compressed and crushed.
In addition to the crushing action described above, slurry particles are also dragged between the valve sealing surfaces in a grinding motion. This grinding action occurs because valve bodies and seats are built with complementary tapers on the sealing surfaces to give the valve a self-alignment feature as the valve body closes against the seat. As the large valve closing force pushes the valve body into closer contact with the seat, the valve body tends to slide down the sealing surface taper by a very small amount. Any crushed slurry particles previously trapped between the sealing surfaces are then ground against these surfaces, resulting in extreme abrasive action.
To limit sealing surface erosion due to this abrasion, valve bodies and seats have in the past been heat-treated to harden and strengthen them. Typical heat treatment methods have included carburizing, as well as hardening by induction heating and flame hardening. All of these hardening processes depend on quenching (i.e., rapid cooling) of the valve components after they have been uniformly heated, preferably slightly above a critical temperature (called the upper transformation temperature).
When a steel object is uniformly heated to a temperature slightly above its upper transformation temperature, all of the steel in the object assumes a face-centered cubic crystal lattice structure known as austenite. When the object is quenched below this temperature, other crystal lattice structures are possible. If quenched uniformly, the other crystal lattice structures tend to appear uniformly throughout the object. But if certain portions of the object are cooled at rates different from those applicable to other portions of the object, then the crystal lattice structure of the cooled object may be non-uniform.
Further, if steel is heated too far above its upper transformation temperature before quenching, its grain structure may be unnecessarily coarsened, meaning that the steel will then be less tough and more brittle after quenching than it would have been if its maximum temperature had been closer to its upper transformation temperature. It is therefore important that heat treatments for a particular steel be applied uniformly when uniform results are desired, and it is further important that maximum temperatures not be so high as to adversely affect the steel's grain structure.
Quenching is preformed primarily to influence the formation of a desirable crystal lattice and/or grain structure in a cooled metal, a grain being a portion of the metal having external boundaries and a regular internal lattice. Quenching may be accomplished, for example, simply by immersion of a heated metal object in water or oil. Certain tool steels may even be quenched by gas (e.g., air or inert gas), but the carbon steels traditionally used for valve seats can not be gas-quenched if they are to develop the hardness, strength and toughness necessary for use in high-pressure valves.
Heat treating of metals has been extensively studied, and many desirable properties may be obtained in metals through elaborate quench and temper protocols that have been experimentally developed. But preferred heat treatments are highly specific to particular alloys, so there may be no single optimal heat treatment for a component such as a valve seat comprising, for example, a high-alloy sealing surface inlay on a carbon steel substrate. Indeed, even the most careful use of heat treatments to favor development of hard sealing surfaces on strong, tough substrates has not proven effective for extending the service life of valves traditionally used for high-pressure abrasive slurries. Thus, engineers have long sought better methods of hardening valve sealing surfaces at acceptable cost.
For example, incorporation of metallic carbides in sealing surfaces has been investigated because some metallic carbides are extremely hard and wear-resistant. But such carbides do not bond well with the low-carbon steels commonly used in high pressure valve seats. Hence, when metallic carbide inlays are applied to such valve seat substrates, they must actually be held in place by some type of cement which itself forms an adequate bond with the valve seat substrate steel.
To facilitate mixing metallic carbides with cement(s), the carbides are made commercially available in powder form. Such powders (e.g., carbides of vanadium, molybdenum, tungsten or chromium) are formed by casting the pure carbides and then crushing them into the desired particle size. A cement (comprising, e.g., cobalt, chromium, and/or nickel) is then added to the crushed carbide powders, but there is little or no opportunity for the cement to alloy with the carbides.
Metallic carbide particles thus bound as an inlay on a steel substrate are called cemented carbides, and they comprise a matrix consisting of a dispersion of very hard carbide particles in the (relatively softer) cement. The resulting cemented carbide inlays are thus not homogeneous, so they do not possess the uniform hardness that would ideally be desired for good abrasion resistance and toughness in valve sealing surfaces. One problem associated with this inhomogeneity becomes evident because the crushing and grinding of slurry particles between valve sealing surfaces during valve closure produces a variety of slurry particle sizes, some so fine that they are smaller than the spacing between the carbide particles in the cemented carbide inlay. These fine slurry particles are very abrasive, and if they can fit between the carbide particles, they can rapidly wear away the relatively soft cement holding the carbide particles in place. Thus loosened (but not actually worn down), the carbide particles can simply be carried away by the slurry stream, leaving the remainder of the inlay cement exposed to further damage by the abrasive slurry. Problems associated with inhomogeneity of cemented carbide inlays may be reduced by choosing relatively high carbide content (e.g., about 85% to about 95%) and sub-micron carbide particle size. Such results have been confirmed by testing according to ASTM B 611 (Test Method for Abrasive Wear Resistance of Cemented Carbides).
Notwithstanding the above problems, cemented carbides, particularly those applied by gas-fueled or electrically-heated welding equipment, have been widely used to reduce abrasion damage in various industrial applications. But weld-applied carbide inlays have not been found acceptable in high pressure valves. This is due in part to a need for relatively high cement content in weld-applied inlays, leading to relatively high porosity inlays having low abrasion resistance and a predisposition to multiple internal stress risers. Low abrasion resistance results from wide spacing of wear-resistant carbide particles, separated by relatively softer cement. And the internal stress risers exacerbate cracking of brittle cemented carbide inlays under the repetitive high-impact loading common in high pressure valves. The result has typically been an increased likelihood of premature (often catastrophic) valve failures. Thus, a long-felt need remains for better technology in valve seat materials that improve valve seat endurance and avoiding an excessive likelihood of cracking.
While such cracks are tolerated in certain applications where the cracks do not significantly affect the performance of the part, the same cannot be said of high pressure valves. On the contrary, cyclic fatigue associated with the repeated large impact loads experienced by these valves magnifies the deleterious effects of cracks and residual stresses that may result from differentials in coefficients of thermal expansion. Premature catastrophic failures of valve bodies and/or seats are a frequent result.
Advances in high pressure plunger pump housings that provide both improved internal access and superior stress reduction are expressed in U.S. Pat. Nos. 6,623,259, 6,544,012 and 6,382,940, which are incorporated herein by reference. As illustrated in FIG. 3, suction and discharge seat bores that accommodate valve seats are typically slightly conically-shaped to facilitate the secure and substantial leak-proof fitment of each valve seat within a bore of pump housing (e.g., by press-fitting a valve seat that has an interference fit with the pump housing).
The press-fitting of valve seats is effective in that the seat is seized in place by the interference fit and an effective high pressure seal is formed between the seat taper and the fluid end taper. However the fluid end housing in the area of the taper is put in tensile stress by interference fit and the seat is put in compressive stress by the interference fit. All metallic structures are substantially incompressible. Thus compressive stress in one area of the seat is relieved as tensile stress in another area of the seat. These applied stresses from the interference fit substantially limits the option of constructing the valve seat from a homogeneously wear resistant material such as tungsten carbide or ceramics.
Standard high-pressure seat designs commonly used in the industry feature seats with a shoulder and a seat taper of 0.75 inches per foot on the diameter. These tapers mate with a similar taper in the receiving fluid chamber of the fluid end. This very “fast” taper is insufficient to retain the seat in a locked position and when the seat is subjected to very high valve loads. Due to such a fast taper, a shoulder is necessary on the seat to prevent the seat from sliding down the taper when the seat is subjected to very high valve loads. Thus, the seat shoulder is exposed to very high downward or axial loads, which results in the very high stresses in the fillet shoulder, as further discussed herein. While the interference fit functions as an effective seal and holding mechanism between the seat and the fluid end housing, the interface is very rigid. Thus all the valve impact loads are transmitted directly to the fluid end housing and the fillet shoulder, increasing stress in the fillet shoulder.